Most Covid-19 convalescent plasma is being screened by one test —

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In the early days of the pandemic, doctors treating Covid-19 patients with convalescent plasma didn’t really know how or whether it was working—they just hoped it was. Some labs tested the blood plasma, donated by people who had recovered from coronavirus infections, for their antibody levels, but they weren’t consistent. “People were using a variety of tests or no test at all,” says Shmuel Shoham, an infectious disease specialist at Johns Hopkins University. If plasma wasn’t tested for antibody levels, well—it was probably better than nothing.

So when US Food and Drug Administration (FDA) finally authorized convalescent plasma for emergency use on Aug. 23, the agency added in a smidge of standardization. The existing research on convalescent plasma suggests that units with a higher level of antibodies work better to fight infections, so the FDA will now require blood banks to test and classify plasma units by high or low antibody counts, called titers.

It’s just a baby step: As of today, the FDA has only picked one test to analyze those antibody levels. But with this added test, doctors can be a little more confident in the donations they’re using to treat patients, and scientists can hopefully learn more about what makes plasma work.

When choosing an antibody test for plasma, the FDA needed one that was ubiquitous enough as to not slow down the donation process, but specific enough to find the antibodies that seem to neutralize Covid-19. It settled on the Ortho Vitros SARS-CoV-2 IgG test, made by Ortho Clinical Diagnostics, a New Jersey-based company that specializes in blood analyzes. (Like plasma itself, Ortho Clinical’s test has an emergency use authorization, not a full FDA approval.)

The chemistry behind it is pretty cool. When you get sick with a virus, your body makes all kinds of antibodies to try to fight it off. Many of them are useless, but some, called neutralizing antibodies, actually stop the virus from infecting cells. Ortho Clinical’s test has a chemical reagent that binds only with antibodies suspected to neutralize the SARS-CoV-2 virus.

That reagent has something else special about it: It’s tagged with a fluorescent chemical that gives off light as it binds with those antibodies. Lab techs (like those at Mayo Clinic, the group that led the biggest network of plasma distribution before FDA authorization) can then measure the light emitted from a sample of plasma to see if it’s high or low titer.

This assay only works on a platform specific to many of Ortho Clinical’s blood tests. It’s a giant machine roughly the size of a dining room table. The good news: It’s entirely automated, and if a lab uses this kind of analyzer to perform other blood tests, it’d simply have to order more of the SARS-CoV-2 assays themselves.

Courtesy of Ortho Clinical Diagnostics

The analyzer.

The bad news is that without this specific platform, labs are out of luck. They’d either have to purchase this entire platform, or ship samples out to another lab that already has one.

Having just one test isn’t a problem right now: The FDA has given all blood banks until Nov. 4 to start labeling their donated plasma, which means they don’t need to use the Ortho Clinical test just yet. A spokesperson for the company declined to comment on the cost or any recent spikes in demand for the Vitros platform.

And the FDA has stated that blood donation centers or labs can use other kinds of antibody titer tests—they just have to take the time to clear it with the Center for Biologics Evaluation and Research first. Jennifer Ficenec, a spokesperson for Blood Centers of America, a network of independent blood donation centers, says that her group isn’t worried about the additional constraint on donations.

Doctors will still be able to give their patients convalescent plasma regardless of whether donated plasma is high or low titer—it doesn’t seem like plasma of any titer will hurt Covid-19 patients. But having it labeled will make it easier to answer some of the questions around plasma’s efficacy. This glow-in-the-dark chemistry is a start to standardizing Covid-19 plasma treatments—and creating some order out of what was previously a bit of scientific chaos.

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